Light petroleum byproduct streams from refinery operations constitute a potentially valuable source of commercially important olefinic feedstocks.
A C.sub.4 fraction of gases from vapor phase cracking of petroleum oil usually contains less than about 15-20 percent of butadiene in admixture with other C.sub.4 hydrocarbons. Likewise, the pyrolysis of butane under conventional conditions converts to butadiene in low yield.
In commercial processes which utilize butadiene it is essential that the butadiene feedstock is provided in concentrated form. For example, in the production of synthetic rubber by copolymerization of butadiene with a monomer such as acrylonitrile, the butadiene feedstock must have a purity of at least about 90 percent or higher.
Various methods have been developed which have as a main object the provision of conjugated diolefinic hydrocarbons such as butadiene in highly concentrated form suitable for commercial polymerization reactions.
U.S. Pat. No. 2,355,392 describes a process for the separation of a low-boiling acyclic diolefin from a mixture of hydrocarbons all of which have boiling points within a narrow range. The process involves (1) subjecting the hydrocarbon mixture to elevated temperature and pressure to promote dimerization of the diolefin, (2) separating the dimer compound from the other components of the resultant product mixture, and (3) subjecting the dimer to depolymerization conditions in the presence of steam to effect conversion of the dimer to the corresponding monomeric product. Optionally, a polymerization catalyst such as oxygen or hydrogen peroxide is employed in the first step, and a depolymerization catalyst such as bauxite is employed in the second step of the process.
U.S. Pat. No. 2,423,179 describes a process for concentrating conjugated dienes containing fewer than seven carbon atoms from mixtures containing them in small proportions which involves (1) subjecting such a mixture to elevated temperatures between about 600.degree.-1100.degree. F. and pressures between about 50-5000 psi in the presence of an oxygen-bearing hydrocarbon derivative having dimerization accelerating properties, thereby forming diene dimers, (2) separating the dimerized products from the residual materials, and (3) depolymerizing the dimerized product to yield the diene monomer in concentrated form by subjecting the dimer to temperatures between about 1200.degree.-1600.degree. F. under partial pressure below atmospheric.
U.S. Pat. No. 2,433,465 describes a process for effecting the separation of a hydrocarbon mixture containing isobutylene, n-butene and butadiene which involves (1) subjecting the mixture to a non-selective thermal polymerization treatment at a temperature between 800.degree.-1000.degree. F. and a pressure of 500-2000 psi, (2) contacting the resulting polymers with a depolymerization catalyst at a temperature between 400.degree.-800.degree. F. to reproduce the isobutylene in monomeric form, (3) separating the reproduced monomeric isobutylene from the unconverted polymers, (4) contacting the unconverted polymers with a depolymerization catalyst at a temperature between 800.degree.-900.degree. F. to reproduce the n-butene in monomeric form, and (5) separating the reproduced monomeric n-butene from the unconverted polymers still remaining.
There remains a need for practical and economical methods for the separation of the components of light hydrocarbon mixtures which are difficult to separate by conventional fractionation means. There is further need for an improved process for deriving concentrated diolefinic hydrocarbon fractions from petroleum refinery light hydrocarbon byproduct streams.
Accordingly, it is an object of this invention to provide a process for the improved separation and utilization of the components of light hydrocarbon mixtures which are difficult to separate.
It is another object of this invention to provide a process for deriving a concentrated diolefinic fraction from a petroleum refinery light hydrocarbon byproduct stream.
It is a further object of this invention to provide a process for converting a C.sub.4 hydrocarbon mixture into high purity butadiene with minimal production of acetylenes.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.